Cellular data overlay system for storing data identifying selected data allocated channels

ABSTRACT

A cellular telephone system includes a packet-switched digital data communication subsystem overlaying a circuit-switched (e.g., voice) communication subsystem, with voice and data services sharing the same voice channels. The voice communication subsystem provides voice services independently of the operation of the data communication subsystem. On the other hand, the data communication subsystem monitors use of the voice channels for voice communication, and, in response thereto, temporarily allocates one or more of the available voice channels for data service. In a preferred embodiment, each data basestation transmits, from time to time, an available-channel signature to identify those, if any, of the data-allocated channels that are not then being used to transmit voice or data. For data communication, a remote unit monitors selected voice channels in the coverage area of the cellular telephone system for available-channel signatures, selects one of the channels (if any) identified by the signature thereon as being available for data transmission, and transmits or receives digital information over that channel.

This is a continuation application of Ser. No. 07/674,660, filed Mar.25, 1991, and now abandoned.

FIELD OF THE INVENTION

This invention relates generally to telecommunication, and moreparticularly to data communication provided in conjunction with voicecommunication over radio channels of cellular telephone systems.

BACKGROUND OF THE INVENTION

A cellular telephone system provides radio communication between astationary base system and remote units, e.g., mobile radio-telephonesinstalled in automobiles. The base system typically includes a switchingsystem, a network of geographically separated basestations, and otherequipment. Each basestation defines a particular cell of the cellularsystem, and has at least one transceiver with the capability of callingto and receiving calls from remote units in the corresponding cell. Sucha cellular telephone system is described in the Bell System TechnicalJournal, January 1979, entitled, "The Cellular Concept," page 15, etseq.

Cellular telephone systems are subject typically to governmentalregulations and approvals, and must comply with standards, such as, forexample, those set forth for the United States of America in EIA RS-553by Electronic Industries Association, Washington, D.C., U.S.A.

A known cellular telephone system allocates to each basestation aplurality of radio channels to carry voice signals (called "voicechannels"), and at least one separate signalling or control radiochannel. In turn, the basestation assigns the allocated voice channelsto remote units for use during voice telephone calls; the assignments ofthe voice channels being made in response to requests from the remoteunits for cellular service for the calls. This preliminary procedure ofrequesting and assigning a voice channel occurs over the controlchannel, and can be referred to as a "hand-shake" transaction.Hand-shake transactions occupy valuable time on the control channel.

Furthermore, because of the nature of cellular speech traffic,substantial voice channel capacity is sometimes idle, e.g., betweentelephone calls. This is even more pronounced during certain periods ofthe day, and during week-ends and holidays.

It has been proposed to provide data communication service in additionto voice service over cellular telephone systems. Two generalalternatives exist in the prior art. In one approach, the cellulartelephone system sends data as well as speech information over the voicechannels, thereby purportedly improving radio channel utilization. Onesuch system is described in U.S. Pat. No. 4,887,265. Sharing channelswith data in accordance with that approach, however, degrades voiceservice insofar as the data competes with voice traffic for channelutilization, and degrades overall system performance in that controlchannels are used to carry both data and voice communication requests.

A second approach avoids interference with voice services byestablishing a separate cellular data system operating over data-onlychannels in frequency bands distinct from those of the voice system. Onesuch system is described in U.S. Pat. No. 4,914,651. This approach,however, does not provide for efficient use of the frequency spectrum,and might be subject to new governmental regulation and approvals, andthe promulgation of new standards for the cellular data system.

SUMMARY OF THE INVENTION

The invention resides in overlaying a packet-switched digital datacommunication subsystem on a circuit-switched (e.g., voice)communication subsystem of a cellular telephone system, with therespective data and voice services sharing the same voice channels. Thevoice communication subsystem provides voice services independently ofthe operation of the data communication subsystem.

On the other hand, the data communication subsystem (or data overlaysystem, as it can be called) monitors use of the voice channels forvoice communication, and, in response thereto, temporarily allocates oneor more of the available voice channels for data service. Bysubordinating data communication in this way, the data service can sharethe voice channels without interfering with voice service.

More specifically, the data communication subsystem has a databasestation in each cell in which data communication service isprovided. The data basestation monitors usage of each data-allocatedvoice channel for voice communication by the voice basestation byanalyzing, e.g., signal characteristics represented by voice-channelutilization signals obtained from the voice basestation or from theairwaves.

For example, the voice-channel utilization signal can be theradio-frequency output signals of the voice basestation, in which casethe data basestation can examine, e.g., the transitions in the signal'sradio-frequency power levels from substantially zero power (indicativeof the absence of voice transmission) to a non-zero power levelindicative of the presence of voice transmission.

Where the voice-channel utilization signal indicates an absence of voicecommunication on the associated voice channel, the data basestationtransmits, from time to time, an available-channel signature to identifythat data-allocated channel as available for data service. Preferably,the available-channel signature is a special signal broadcasted on theavailable data-allocated channels not then being used to transmit voiceor data.

For data communication, a specially-equipped data remote unit monitorsvoice channels in the coverage area of the cellular telephone system foravailable-channel signatures, selects one of the channels (if any)identified by the signature thereon as being available for datacommunication, and transmits or receives digital information over thatvoice channel.

On initiation of a "voice" telephone call on that channel, however, anytransmission (whether of data or a carrier signal) by the data overlaysystem over the channel abruptly terminates, preferably even though,e.g., the data transmission has not yet been completed. This preventsthe data service from interfering with voice service provided by thecellular telephone system. If the data transmission is interrupted priorto completion of transmission of the data, the remote unit repeats thechannel selection process, and continues data transmission on anothervoice channel.

Preferably, the remote unit maintains a list or look-up table stored inmemory of the data-allocated channels associated with each cell. Thelook-up table can be static or, preferably, dynamic, with up-datesfurnished (i.e., broadcasted) from time to time by the databasestations. Accordingly, during channel selection, in order toidentify an available data-allocated channel, the remote unit canmonitor only the more likely available voice channels identified by theupdated data-allocated-channel list to find an available-channelsignature, rather than, for example, all voice channels for theparticular cell.

Accordingly, the invention improves channel utilization by sending dataon voice channels. Moreover, the invention assigns a higher priority tovoice service and accordingly controls data service by limiting channelaccess by the latter service to only those times when the channels arenot being used for voice service. This approach avoids interference withprimary voice service, yet it provides materially greater utilization ofthe overall channel capacity of the system by making use of otherwiseinactive interludes in the voice channels.

Moreover, the invention eliminates hand-shaking over the separatecontrol channels when data transmission is to take place, since a remoteunit can "seize" and transmit data over an available data-allocatedchannel, and need not request and await assignment of a channel from thebasestation prior to data transmission.

Advantageously, the data overlay system in accordance with the inventioncan usually be added to existing telephone cellular systems with nodegradation of voice communication services, and with no modificationsof existing voice basestations. In addition, the data overlay system caneconomically support a wide range of data communication applicationswith accurate and reliable transmission at a sufficiently high datathrough-put to accommodate the needs of a sizable number of concurrentusers.

Finally, it is expected that the data overlay system in accordance withthe invention can be implemented without requiring the promulgation ofnew standards.

BRIEF DESCRIPTION OF THE DRAWING

The above and further advantages of the invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which:

FIG. 1 is a block diagram of a cellular telephone system provided with adata overlay system in accordance with the invention;

FIG. 2 is a detailed block diagram of the voice basestation, databasestation and data remote unit of FIG. 1;

FIG. 2a is a detailed block diagram of the central processing unit 58 ofthe data basestation of FIG. 2;

FIGS. 3 and 4 are flowcharts depicting the respective operation of thedata remote unit and the data basestation shown in FIG. 2 duringremote-unit-to-basestation transmission of data packets;

FIG. 4a is a block diagram of an available-channel signature;

FIGS. 5 and 6 are flowcharts depicting the respective operation of thedata remote unit and the data basestation shown in FIG. 2 duringbasestation-to remote-unit transmission of data packets; and

FIG. 7 is a flowchart depicting another, optional technique of assuringthat data service does not interfere with voice service in the cellulartelephone system of FIG. 1.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

FIG. 1 shows a cellular telephone system 10 including a datacommunication subsystem 12 overlaying a voice communication subsystem14. The voice communication subsystem 14 has a plurality of voicebasestations 16, each associated with a coverage area or cell 18 of thecellular telephone system 10.

Each voice basestation 16 provides voice communication services betweenone or more conventional voice remote units 20a, which are equipped forvoice communication and present within its coverage area, and aconventional telephone network 22. A voice switching system 26 connectedto the basestations 16 provides circuit switching for calls from thevoice remote units 20a to the telephone network 22, and vice versa.

The data communication subsystem 12 has a plurality of data basestations36, each associated with one of the cells 18 of the cellular telephonesystem 10. Each data basestation 36 can provide data communicationservices between data remote units 20b, which are equipped for datacommunication, when such remote units 20b are present within itscoverage area and a conventional data or computer network 42. A dataswitching system 46 connected to the data basestations 36 providespacket switching for calls from the data remote units 20b to the datanetwork 42, or vice versa.

For data transmission, the remote units 20b establish communications viaselected basestations 36 serving the cells in which the remote units 20bare located. As the remote units 20b move from those cells 18 to others,communications with those selected basestations 36 are terminated, andthe remote units 20b re-establish communications via other basestations36.

The remote units 20a, 20b are depicted as mounted in automobiles,although other portable radio telephones, such as, for example,hand-held units and even non-portable wireless terminals (for example,security and control/monitoring units), fall within the scope of theinvention.

In accordance with the invention, the data communication subsystem 12(i) shares many of the same voice channels with the voice communicationsubsystem 14, and yet does not interfere with voice service of thecellular system 10, i.e., is transparent to users placing calls thereinfor voice communication, and (ii) can often be implemented in cells 18having a voice basestations 16 already installed without requiringmodification or retrofit of those voice basestations 16.

FIG. 2 depicts the components of the cellular telephone system 10associated with one of the cells 18. Specifically, the voice basestation16 contains a number of conventional transceivers 50, each of whichbeing tunable to a particular voice channel, and capable of duplexoperation via antenna 52 for voice communication over that channel inaccordance with cellular telephone standards. The voice basestation 16also contains audio lines 53 for connecting the transceivers 50 to thevoice switching system 26 (FIG. 1), and thence to an existing land linetelephone network 22 for voice service. The voice basestation 16 alsocan include other conventional components, which need not be describedherein since they are not involved in the present invention.

The data basestation 36 includes a number of transceivers 54, each ofwhich being tunable to communicate over an assigned voice channel andcapable of duplex operation via antenna 60 for data communication overthat channel. The transceivers 54 are each connected by a modem 56 to acentral processing unit ("CPU") 58. The CPU 58 is connected to the dataswitching system 46 and, thereby, to the data network 42.

The basestation 36 receives a radio frequency signal containing digitaldata over one of the allocated voice channels. The particulartransceiver 54 that is tuned to that voice channel converts the receivedsignal into a baseband signal. The modem 56 coupled with thattransceiver 54 demodulates the baseband signal to recover the datacontained therein. The data is provided to the CPU 58.

Similarly, for transmission of data, the CPU 58 of the basestation 36receives the data via the switching system 46 from the data network 42,or generates control or status data, and, in either case, applies thedata to one of the modems 56 for modulation thereof in accordance withany suitable modulation scheme, thereby generating a baseband signal.The transceiver 54 coupled to that modem 56 modulates a carrier signalwith the baseband signal, and transmits the resulting modulatedradio-frequency signal via the antenna 60. The CPU 58 controls theoperation of the transceiver 54 and modem 56.

The data basestation 36 also employs a scanner 62, which receives avoice-channel-utilization signal over line 64 from each of thetransceivers 50 of the voice basestation 16. That signal indicateswhether the voice channels associated with the transceivers 50 of thevoice basestation 16 are being used for voice communication.

For example, the voice-channel-utilization signal can designate theinitiation and termination of voice telephone calls over the voicechannel by representing the power levels or transitions, spectra, orother indicia of voice transmission from the transceiver 50.

More specifically, the voice-channel utilization signal can indicate theuse of the voice channel by representing radio-frequency ("RF") powerlevels or transitions between power levels in an output of the voicebasestation 16, or by representing control signals used within the voicebasestation 16, such as those used to control the variable-gainamplifiers typically found in the terminal stages of transmitterstransceivers 50 thereof.

Another technique is to obtain and demodulate the voice basestation RFoutput signals, and check for the presence of certain controlinformation (e.g., supervisory audio tones) typically included thereinin accordance with the aforementioned standards and indicative of voicecommunication.

Many commercially available voice basestations have external ports whichpermit access to one or another of these signals, in which case, thevoice basestations need not be modified to provide the aforementionedvoice-channel utilization signals to the data basestations.

Alternatively, rather than receiving the voice-channel-utilizationsignal over line 64, the scanner 62 of the data basestation 36 caninstead intercept broadcasted voice communication signals on the voicechannels, and use, e.g., the RF power levels or signal spectra orcontents of the intercepted signals as the voice-channel utilizationsignals. Such non-intrusive approaches obviously require neithermodification of the voice basestations, nor access to signals within thevoice basestations.

The data basestation 36 also has a duplicate detector 65, whichdetermines whether a received data packet is a duplicate of a previouslyreceived packet. The duplicate detector 65 can be implemented as aseparate device, or can be implemented as a software routine within theCPU 58.

FIG. 2a shows the components of CPU 58. CPU 58 has a processor 72,memory 74, clock or timing circuit 76 and one or more input/output("I/O") devices 78. These components 72 through 78 are interconnected bya bus 82, and, along with other conventional components, can beimplemented, for example, in a single integrated circuit. Where theduplicate detector routine is resident within the CPU 58, the routinecan be stored in memory 74, and run by processor 72.

Returning to FIG. 2, the data remote unit 20b is a radio-telephoneequipped for data communication. The data remote unit 20b includes atransceiver 84, which is tunable to communicate over any of a pluralityof voice channels, and an antenna 92 capable of duplex communication inaccordance with cellular telephone standards. The transceiver 84 isconnected via a modem 86 to a conventional data terminal 88, e.g., aportable or other computer.

For data transmission from the data remote unit 20b, a controller 94passes the data to the modem 86, at which the data is modulated pursuantthe same modulation scheme used by the data basestation 36. The modem 86provides the modulated data to the transceiver 84, which mixes themodulated data with a radio-frequency carrier for transmission via theantenna 92 over a voice channel. The transmitted data can be control orstatus data generated by the controller 94, or message data generated bythe data terminal 88. The controller 94 controls operation of the othercomponents of the data remote unit 20b in accordance with a routinestored in a memory 96.

On the other hand, when the remote unit 20b receives a broadcast datasignal over a voice channel, the transceiver 84 converts the receivedsignal into a baseband signal. The modem 86 then demodulates thebaseband signal so as to form data packets, which are then passed to thecontroller 94. The received data packets can be control or statuspackets for use by the controller 94, or can be message packets, whichare routed by the controller 94 to the data terminal 88. A duplicatedetector 98 ascertains whether the received data packets are duplicatesof ones previously received.

The data remote unit 20b further contains a scanner 100, which is usedduring channel selection as described hereinafter by causing thetransceiver 84 to tune to a plurality of channels to identify availablechannels allocated for data communication. The purpose and operation ofthe scanner 100, as well as the procedure for selection of voicechannels for data communication, will be described in greater detailhereinbelow.

An illustrative embodiment of the operation of the data remote unit 20band the data basestation 36 will be described next. It should beunderstood at the outset that the operation of the data remote unit 20bis controlled preferably by a program stored in memory 96 and executedby controller 94. Likewise, the operation of the data basestation 36 iscontrolled preferably by a program stored and executed by the CPU 58.

Remote-To-Base Data Transmission

FIGS. 3 and 4 show respective data remote unit and data basestationoperation during remote-to-base transmission of data in accordance withan illustrative embodiment of the invention. In FIG. 3, in start block102, the remote unit 20b performs, e.g., self-diagnostic tests. Then,for data communication, in block 104, the remote unit 20b performschannel selection.

For channel selection, the remote unit 20b determines which of thecontrol channels of all voice basestations 16 in the cellular telephonesystem 10 carries the strongest signal, and then identifies the voicechannels associated with that control channel. This process assuresthat, as the remote unit 20b moves relative to the various cells 18,data transmission will occur with data basestations 36 and over voicechannels that can carry an adequately strong signal for reliable datacommunication.

Specifically, the scanner 100 in the remote unit 20b causes thetransceiver 84 to scan a plurality of control channels that are storedin a control-channel-database in memory 96. For each control channel,the transceiver 84 measures the received-signal-strength and provides areceive signal strength indicating ("RSSI") signal to the controller 94.The controller 94 selects the control channel having the strongestsignal and then uses a voice-channel database also stored in memory 96to identify all voice channels associated with the identified controlchannel, and, thus, associated with the voice and data basestations 16,36 of the cell 18 that is associated with the identified controlchannel.

Preferably, the voice channel database is a listing of specific"data-allocated channels" that have been allocated for datacommunication by the cellular telephone system 10 to the databasestations 36. Thus, the voice-channel database is a look-up tablethat identifies the data-allocated channels for use by the remote unit20b.

This look-up table can be either static or, preferably, dynamic, withupdates furnished from time to time by the data basestation 36. Whendynamic, the data-allocated channels identified in the look-up table canbe reduced in number so as to be only those channels which are not then,at the time of the up-dates, as indicated by thevoice-channel-utilization signals, being used for voice communication.Methods for providing the up-dates will be described shortly.

In blocks 106, 108, the remote unit 20b determines which of thedata-allocated channels identified in the stored voice channel databaseare available, at that time, for data communication. Specifically, inblock 106, the remote unit 20b tests whether a first of thedata-allocated channels carries an "available-channel signature."

Preferably, the available-channel signature is a special digital signalgenerated by the modem 56 in response to commands from the CPU 58, andbroadcasted by the data basestation 36 on any data-allocated channelwhich is not at that time being used for voice or data communication.

As shown in FIG. 4a, the available-channel signature can have a firstfield of digital data providing identification information that will berecognized by the controller 94 as indicating that this particularsignal is an available-channel signature, thus indicating that thechannel over which it is being sent is available for data transmission.Preferably, and as shown, the available-channel signal also has a secondfield of digital data providing the voice-channel-database-updateinformation that was mentioned above.

(Alternatively, the update information for the voice-channel databaseneed not be contained in the available-channel signal, but, rather, canbe furnished by separate update signals sent by the data basestation 36,e.g., periodically or whenever a change is to be made to the voicechannel database.)

Returning to FIG. 3, if the first data-allocated channel does not carryan available-channel signature, then, in block 108, then the remote unit20b switches to a next data-allocated channel, and repeats the "detectsignature" test of block 106. This process continues until a channelcarrying the signature is identified, in which case the transceiver 84has been tuned to a voice channel available for data transmission.

Accordingly, in the preferred embodiment, the remote unit 20b itselfselects an available data-allocated channel for data transmissionwithout involving a hand-shake transaction (e.g., an exchange of controlpackets on a control channel) with potential basestations.

Subsequently, in block 110, the remote unit 20b sends a data packet overthe identified, available data-allocated voice channel.

In block 112, the remote unit 20b tests whether, within a period T1after the data packet has been transmitted, the transceiver 84 stopsreceiving the available-channel signature over the identified dataallocation channel used for that data packet transmission. Normally, inresponse to receipt of a data packet from a remote unit 20b, as will bedescribed below, the data basestation 36 disables the available-channelsignal on that channel. Thus, this test provides a first indication ofwhether the data basestation 36 has in fact received the data packet.This is analogous to the "busy idle bit" routine specified in the abovereferenced EIA RS-553 standards.

If no loss of the available-channel signature is detected within thetime T1, the remote unit 20b then returns to block 104 for channelselection. On the other hand, if a loss of the available-channelsignature is detected within that period of time, the remote unit 20bnext goes to block 114.

In block 114, the remote unit 20b checks whether the transceiver 84 hasreceived an acknowledgement from the data basestation 36. Usingacknowledgements to confirm receipt of data packets finds utility hereinbecause, even though there has been a detected loss of theavailable-channel signature, that loss of signature may have resultedfrom receipt by the basestation 36 of a data packet from another remoteunit 20b, and not from the one whose operation is being describedherein. Accordingly, if the remote unit 20b receives an acknowledgement,then the data basestation 36 has in fact received the data packetbroadcasted by the remote unit 20b, and the remote unit 20b next returnsto block 104 for channel selection.

On the other hand, if no acknowledgement is received, then, in block116, the remote unit 20b again checks whether a available-channelsignature has been received from the data basestation within a time T2,thus indicating that the same voice channel can be used for further datatransmission. If the signature is not detected, then the remote unit 20breturns to block 104, and channel selection is repeated.

On the other hand, if the signature is detected within T2, then in block118, the remote unit 20b resends the data packet and, in block 122,increments a count to indicate that the data packet has been resent.After incrementing the count, the remote unit 20b then, in block 124,tests whether the count is greater than a preselected number, N1. If thecount variable is not greater than the N1 limit, then the remote unit20b rechecks for loss of signature in block 112, as indicated by thecircled "B" in the drawing. However, if the count variable is greaterthan the N1 limit, then the remote unit 20b repeats the channelselection step at block 104.

In certain applications, acknowledgements of transmissions are notrequired and the acknowledgement steps just described can be omitted, inwhich case, after sending the data packet in block 110, the remote unit20b returns to block 104.

FIG. 4 depicts data basestation operation during remote-to-basetransmission. As represented by block 132, the data basestation 36initially performs, e.g., self-diagnostic tests. Then, in block 134, thebasestation 36 checks whether a first of the data-allocated channels isbeing used for data or voice communication. The data-allocated voicechannels are identified in a data-allocated voice channel look-up tablemaintained in memory 74.

More specifically, the CPU 58 determines whether the data basestation 36is itself using the voice channel for data communication, and determineswhether the basestation 36 has received a voice-channel-utilizationsignal indicating that the voice channel is being used by the voicebasestation 16 for voice communication.

If that channel is being so used, then, in block 136, the databasestation 36 switches to a next of the data-allocated voice channelsand repeats the test of block 134 for the next channel.

On the other hand, if the data basestation 36 determines that thedata-allocated voice channel is not being used at that time for eithervoice or data transmission, the basestation 36 next, in block 138,enables an available-channel signature indicating that channel asavailable for data communication. More specifically, the CPU 58 causesthe transceiver 54 to broadcast, e.g., the special digital signal onthat data-allocated channel to identify that channel as being availablefor data transmission.

Further in block 138, the data basestation initializes and starts a datachannel use timer 72a, which is shown in FIG. 2a as a part of theprocessor 72. The timer 72a responds to timing signals from the clock76, and measures the total duration of the use of the channel for datatransmission. The purpose of the timer 72a will be described below.

The data basestation 36 next, in block 142, checks whether thetransceiver 54 has received a packet from any remote unit 20b. If nopacket has been received, the data basestation 36 waits until a datapacket is received by continuously performing the test of block 142. Onreceipt of a data packet, in block 144, the data basestation 36 disablesthe available-channel signature, i.e., stops broadcasting the signature,as mentioned hereinabove during the description of remote unitoperation.

In block 146, the data basestation 36 sends an acknowledgement ofreceipt of the data packet.

After receiving the data packet, the data basestation 36 determineswhether the data packet is a duplicate of one that was earlier received.This test is performed in block 148. If the packet is a duplicate, thedata basestation 36 ignores the duplicate packet per block 152. On theother hand, if the data packet is not a duplicate, then, in block 154,the data basestation 36 sends the packet to the data switching system 46for routing to its destination.

In either case, the data basestation 36 next, in block 156, once againenables the available-channel signature, and then the above-describedprocedure is repeated, as indicated by the circled "C" in the drawing,starting with block 142.

As explained above, the cellular telephone system 10 provides dataservices in such a way as to assuredly not interfere with voiceservices. To that end, the data basestation 36 takes certain measuresduring remote-to-base transmission to prevent such interference byappropriately controlling data service. These measures will be describednext.

After the data basestation enables the signature and starts the datachannel use timer in block 138, in addition to the test performed inblock 142, the data basestation 36 performs parallel tests in block 162and 164. Based on the outcomes of these tests, the CPU 58 causes thebasestation 36 to step through the operations of the blocks 142 through156 as just described, or terminates, at any time, the operations ofthose blocks.

In block 162, the data basestation 36 continuously tests whether thevoice channel is being used for voice communication. The initiation ofvoice communication is determined by the data basestation 36 through theuse of scanner 62 and CPU 58 respectively obtaining and recognizing asignal (described hereinabove as the voice channel utilization signal)from the voice basestation 16, which indicates that voice communicationhas begun.

On initiation of voice communication, the data basestation 36immediately, as indicated in block 166, disables the available-channelsignature, stops sending any acknowledgement that is being sent in block146, and turns off the transmitter 54 so that not even the carrier isbeing sent. Afterwards, as indicated by the circled "D" in the drawing,the data basestation 36 switches to the next channel at block 136. Onthe other hand, as mentioned above, if the test of block 162 isnegative, the test is repeated successively until such time as a voicechannel utilization is detected.

In block 164, the data basestation 36 checks whether the data channeluse timer indicates a duration greater than or equal to a preselectedtime, designated T3. It should be apparent from the above descriptionthat the data channel use timer 72a measures the total duration of useof a particular data channel from the time that the available-channelsignature is enabled in block 138.

If that time exceeds the maximum duration T3, then the data basestation36 proceeds to block 166, where the signature is disabled, andtransmission of the acknowledgment is stopped, both as described above.On the other hand, if the channel-use-time variable has not reached theT3 limit, the test of 164 is successively performed.

Accordingly, the data basestation 36 controls data service duringremote-to-base communication to assure that the transmission of datapackets over voice channels does not interfere with voice services.

Base-to-Remote Data Transmission

FIGS. 5 and 6 show respective data remote unit and data basestationoperation during base-to-remote transmission of data in accordance withan illustrative embodiment of the invention. In FIG. 5, in start block202, the remote unit 20b performs, e.g., self-diagnostic tests. Then,for data communication, in block 204, the remote unit 20b performschannel selection in the same manner as described above with respect toblock 104 of FIG. 3.

Subsequently, in blocks 206 and 208, the data remote unit 20b determineswhich of the identified data-allocated channels is available for datacommunication by testing for available-channel signatures, as describedabove with respect to blocks 106 and 108 of FIG. 3.

In block 210, the data remote unit 20b sends a RECEIVE-READY signal overthe identified, available voice channel. The RECEIVE-READY signalindicates that the data remote unit 20b is ready to receive data fromthe data basestation 36, and designates to the data basestation 36 thatthe channel over which it receives the RECEIVE-READY signal should beused for sending data to the remote unit 20b.

Accordingly, in the preferred embodiment, the remote unit 20b selectsthe voice channel to be used for data communication not only forremote-to-base transmission, but also for base-to-remote transmission.

Subsequently, in block 212, the remote unit 20b tests whether, within aperiod T4 after sending the RECEIVE-READY signal, the transceiver 84stops receiving the available-channel signature over the voice channelon which the remote unit 20b had sent the RECEIVE-READY signal. Asbefore with respect to block 112 of FIG. 3, this test provides anindication of whether the data basestation 36 has received thetransmitted data, which this time is the RECEIVE-READY signal, since thedata basestation 36 disables the signature on receipt of the data, aswill be described below. If no loss of the signature is detected, thenthe remote unit 20b returns to block 206 to resend the RECEIVE-READYsignal.

However, if a loss of the signature is detected, then, in block 214, theremote unit 20b checks whether the transceiver 84 has received a datapacket from the basestation 36. If no packet is received, the remoteunit 20b returns to block 208 and switches to the next channel.

On the other hand, if a data packet is received by the transceiver 84,then, in block 216, the remote unit 20b starts the receiptacknowledgment process by detecting whether the available-channelsignature reappears within a preselected time T5 on the voice channelover which the data packet was received. If no signature is detected,the acknowledgment cannot be sent over that voice channel, the remoteunit 20b returns to block 208 and switches to the next channel, andrepeats the procedures just described.

If the signature is detected, then, in block 218, the remote unit 20bsends an acknowledgment to the data basestation 36 to acknowledgereceipt of the data packet. As above, loss of the signature is detectedin block 219 to indicate receipt of the acknowledgement by thebasestation 36. If the signature disappears within time T4, then theremote unit 20b goes to block 220. Otherwise, the acknowledgment isresent at block 218.

(Preferably, block 219 also has a counter (not separately shown) tolimit the number of times the acknowledgement is sent. Once that limitis met, the remote unit 20b returns to block 208 and switches to thenext channel.)

Next, the remote unit 20b tests whether the received data packet is aduplicate in block 220, and either ignores the packet in block 222 if itis a duplicate, or else sends the non-duplicate packet in block 224 tothe data terminal 88 (FIG. 2). This procedure of testing whether thedata packet is a duplicate is analogous to that of blocks 148, 152, and154 in FIG. 4. After either ignoring the packet as a duplicate, orsending the packet to the data terminal 88, the remote unit returns toblock 208, switches to a next channel, and then repeats the "detectsignature" test of block 206.

FIG. 6 depicts data basestation operation during base-to-remotetransmission. As represented by block 232, the data basestation 36initially performs, e.g., self-diagnostic tests. Then, in block 234, thebasestation 36 checks whether a first of the data-allocated channels isbeing used for data or voice communication.

If that channel is being so used, then, in block 236, the databasestation 36 switches to a next of the data-allocated voice channels,and repeats the test of block 136 for the next channel.

On the other hand, if the data basestation 36 determines that thedata-allocated voice channel is not being used at that time for eithervoice or data transmission, the basestation 36 next, in block 238,enables an available-channel signature indicating that channel asavailable for data communication.

Further in block 238, the data basestation initializes and starts a datachannel use timer 72a, which is shown in FIG. 2a as a part of theprocessor 72.

The procedures of blocks 234 through 238 are analogous to thosedescribed for blocks 134 through 138 of FIG. 4. Accordingly, a furtherdescription of the operation of the basestation 36 in blocks 234 through238 can be had by reference to the relevant paragraphs above.

The data basestation 36 next, in block 242, checks whether thetransceivers 54 have received a RECEIVE-READY signal from any remoteunit 20b. If no such signal has been received, the data basestation 36waits until a RECEIVE-READY signal is received by continuouslyperforming the test of block 242. On receipt of a data packet, in block244, the data basestation 36 disables the available-channel signature,i.e., stops broadcasting the signature, as mentioned hereinabove duringthe description of remote unit operation.

Subsequently, in block 246, the data basestation 36 checks whether adata packet to be sent to the remote unit 20b is available in memory 74of CPU 58. If such a data packet is available, then, in block 252, thebasestation 36 causes the transceiver 54 that is tuned to the voicechannel over which the RECEIVE-READY signal was received to transmit theavailable data packet over that voice channel. Then, the signature isre-enabled.

Then, in block 254, the data basestation 36 tests whether it hasreceived an acknowledgment from the remote unit 20b to which the datapacket was sent, as described above with respect to blocks 216 and 218of FIG. 5. If an acknowledgment has been received, then, in block 255,the basestation 36 disables the signature for time T7.

After block 255, or if no data packet is available per the test of block246, then, in block 256, the basestation 36 once again enables thedata-allocated signature over that voice channel, and, as indicated bythe circled "H" in the drawing, returns to the "RECEIVE-READY SIGNAL"test of block 242. Thereafter, the above-identified procedure isrepeated to permit the sending of any remaining, available data packetshaving the same remote unit 20b as their destination.

On the other hand, if no acknowledgment is received from the remote unit20b to which the data packet was transmitted, then, in block 258, thebasestation 36 resends the data packet. Thereafter, the basestation 36increments a count in block 259, and tests whether the count has reacheda preselected number, N2, in block 260. If it has, then the basestation36 proceeds to block 256, at which the signature is enabled, asdescribed above. Otherwise, if the count has not reached the N2 limit,the basestation 36 enables the signature in block 261, and then returnsto block 254, at which it again checks whether an acknowledgment hasbeen received.

As explained above, the cellular telephone system 10 provides dataservice in such a way as to assuredly not interfere with voice service.To that end, the data basestation 36 again takes certain measures duringbase-to-remote transmission to prevent such interference byappropriately controlling data service. These measures will be describednext.

After the data basestation enables the signature and starts the datachannel use timer in block 238, in addition to the test performed inblock 242, the data basestation 36 performs parallel tests in block 262and 264. Based on the outcomes of these tests, the CPU 58 causes thebasestation 36 to step through the operations of blocks 242 through 260or 261, as just described, or terminates, at any time, the operations ofthose blocks.

In block 262, the data basestation 36 continuously tests whether thevoice channel is being used for voice communication. The initiation ofvoice communication is determined by the data basestation 36 through theuse of scanner 62 and CPU 58 respectively obtaining and recognizing asignal (described hereinabove as the voice channel utilization signal)from the voice basestation 16, which indicates that voice communicationhas begun.

On initiation of voice communication, the data basestation 36immediately, as indicated in block 266, disables the available-channelsignature, stops sending any data packet that is in process of beingsent in block 252 or 258, and turns off the transmitter 54. Afterwards,as indicated by the circled "G" in the drawing, the data basestation 36switches to the next channel at block 236. On the other hand, asmentioned above, if the test of block 262 is negative, the test isrepeated successively until such time as voice channel utilization isdetected.

In block 264, the data basestation 36 checks whether the data channeluse timer indicates a duration greater than or equal to a preselectedtime, designated T6. It should be apparent from the above descriptionthat the data channel use timer 72a measures the total duration of useof a particular data channel from the time that the available-channelsignature is enabled in block 238.

If that time exceeds the maximum duration T6, then the data basestation36 proceeds to block 266, where the signature is disabled, transmissionof data packets is stopped, and the transmitter 54 is disabled so thatno signal can be sent that might interfere with voice communication, allas described above. On the other hand, if the channel-use-time variablehas not reached the T6 limit, the test of 264 is successively performed.

Accordingly, the data basestation 36 controls data service during bothremote-to-base and base-to-remote communication to assure that thetransmission of data packets over voice channels does not interfere withvoice services.

Another, optional technique of assuring that the data service will notinterfere with voice service entails limiting data transmission on avoice channel by the remote unit 20b to a preselected maximum duration.In other words, the remote unit 20b times each data call, and terminatedafter a selected measured duration.

FIG. 7 illustrates this technique. In block 300 of that drawing, theremote unit 20b starts sending either a data packet in block 110 of FIG.3 or a RECEIVE-READY signal in block 210 of FIG. 5, as described above,over an available data-allocated channel. Immediately upon startingtransmission, the remote unit 20b, in block 302, starts a transmissiontimer. For this, the controller 94 has a clock or timing circuit 94a, asshown in FIG. 2.

In block 304, at the end of a preselected period of time, (i.e., whenthe timer 94a has reached a preselected duration, T7), the remote unit20b terminates transmission on that voice channel.

Afterwards, in block 306, the remote unit 20b checks whether datatransmission had been completed before T7 is reached and transmissionterminated. If transmission had not been completed, then, in block 308,remote unit 20a selects a further available data-allocated voicechannel, and repeats the data transmission. On the other hand, iftransmission is completed within time T7, then this technique does notaffect transmission.

The foregoing description of the data remote unit 20b applies as well toa remote unit 20c, shown in FIG. 1, which contains both the componentsof the voice remote unit 20a and those of the data remote unit 20b thatare needed for voice and data communication. Thus, for data transmissionover the cellular telephone system 10, either a data-only remote unit20b or a hybrid, data and voice remote unit 20c can be used.

The foregoing description has been limited to a specific embodiment ofthis invention. It will be apparent, however, that variations andmodifications may be made to the invention, with the attainment of someor all of its advantages. Therefore, it is the object of the appendedclaims to cover all such variations and modifications as come within thetrue spirit and scope of the invention.

What is claimed is:
 1. A cellular telephone system comprising:A) a voicecommunication subsystem including at least one voice basestation forproviding voice service over a plurality of voice channels; and B) adata communication subsystem including at least one data basestation forproviding data service over one or more of said voice channels; C)wherein said voice basestation includes a voice basestation transceiverfor transmitting a control signal over at least one control channel anda plurality of voice signals over said voice channels; said voicecommunication subsystem providing said voice service independently ofthe data service of said data communication subsystem; D) wherein saiddata communication subsystem includes basestation monitoring means formonitoring the voice service of said voice communication subsystem, adata basestation transceiver for transmitting data signals over saidvoice channels, and processor means responsive to said basestationmonitoring means for controlling said data communication transceivermeans so as to avoid having said transmitted data signals interfere withsaid voice service; E) wherein said basestation monitoring meansmonitors use of said voice channels by said voice communicationsubsystem to identify the voice channel or channels which are not beingused for voice service and, in response thereto, said data communicationsubsystem provides data service on one or more of said identified voicechannels, and F) wherein said basestation monitoring means monitors useof said identified voice channels for voice service by said voicecommunication subsystem while data service is being provided, and, inresponse thereto, said data communication subsystem terminates dataservice on any of said identified voice channels upon commencement ofuse by said voice communication subsystem of that channel for voiceservice.
 2. A cellular telephone system comprising:A) a voicecommunication subsystem including at least one voice basestation forproviding voice service over a plurality of voice channels; and B) adata communication subsystem including at least one data basestation forproviding data service over one or more of said voice channels; C)wherein said voice basestation includes a voice basestation transceiverfor transmitting a control signal over at least one control channel anda plurality of voice signals over said voice channels; said voicecommunication subsystem providing said voice service independently ofthe data service of said data communication subsystem; D) wherein saiddata communication subsystem includes basestation monitoring means formonitoring the voice service of said voice communication subsystem, adata basestation transceiver for transmitting data signals over saidvoice channels, and processor means responsive to said basestationmonitoring means for controlling said data communication transceivermeans so as to avoid having said transmitted data signals interfere withsaid voice service; and E) wherein said basestation monitoring meansmonitors use of said voice channels by said voice communicationsubsystem to identify the voice channel or channels on which voiceservice has begun, and, in response thereto, said data communicationsubsystem terminates data service on said voice channels identified asproviding voice service.
 3. The cellular telephone system in accordancewith claim 2, wherein said basestation monitoring means includes meansfor scanning power levels of signals produced by said voicecommunication subsystem.
 4. The cellular telephone system in accordancewith claim 3, wherein said data communication subsystem identifiesradio-frequency channels on which circuit-switched service has begun bymonitoring circuit-switched service transmissions in said voicecommunication subsystem.
 5. A cellular telephone system comprising:A) avoice communication subsystem including at least one voice basestationfor providing voice service over a plurality of voice channels; and B) adata communication subsystem including at least one data basestation forproviding data service over one or more of said voice channels; C)wherein said voice basestation includes a voice basestation transceiverfor transmitting a control signal over at least one control channel anda plurality of voice signals over said voice channels; said voicecommunication subsystem providing said voice service independently ofthe data service of said data communication subsystem; D) wherein saiddata communication subsystem includes basestation monitoring means formonitoring the voice service of said voice communication subsystem, adata basestation transceiver for transmitting data signals over saidvoice channels, and processor means responsive to said basestationmonitoring means for controlling said data communication transceivermeans so as to avoid having said transmitted data signals interfere withsaid voice service; and E) wherein said data communication subsystemfurther includes at least one remote unit, said basestation transceivertransmitting, from time to time, an available-channel signature to saidremote unit to identify those, if any, of the voice channels that arenot then being used to transmit voice signals.
 6. The cellular telephonesystem in accordance with claim 5, wherein said remote unit includesremote unit monitoring means for monitoring each of a plurality ofselected voice channels for an available-channel signature, controllermeans coupled with and responsive to said remote unit monitoring meansfor selecting one of the channels, if any, identified by the signaturethereon as being available for data transmission, and remote unittransceiver which transmits or receives data over that channel.
 7. Thecellular telephone system in accordance with claim 6, wherein said databasestation transceiver transmits data over said selected channel tosaid remote unit.
 8. A cellular telephone system comprising:A) a voicecommunication subsystem including at least one voice basestation forproviding voice service over a plurality of voice channels; and B) adata communication subsystem including at least one data basestation forproviding data service over one or more of said voice channels; C) aplurality of cells; D) wherein said voice basestation includes a voicebasestation transceiver for transmitting a control signal over at leastone control channel and a plurality of voice signals over said voicechannels; said voice communication subsystem providing said voiceservice independently of the data service of said data communicationsubsystem; E) wherein said data communication subsystem includes (i)basestation monitoring means for monitoring the voice service of saidvoice communication subsystem, (ii) a data basestation transceiver fortransmitting data signals over said voice channels, and (iii) processormeans responsive to said basestation monitoring means for controllingsaid data communication transceiver means so as to avoid having saidtransmitted data signals interfere with said voice service; F) whereinsaid voice communication subsystem includes a voice basestation in eachsaid cell, each said voice basestation having an associated controlchannel, G) said data communication subsystem includes a databasestation associated with each said voice basestation, and at leastone remote unit, each said data basestation including said basestationmonitoring means for monitoring the voice service of the associatedvoice basestation, and H) said processor means, in response to saidbasestation monitoring means, controlling data service so as to avoidinterfering with voice service.
 9. The cellular telephone system inaccordance with claim 8, wherein said basestation monitoring meansmonitors use of said voice channels by said voice communicationsubsystem to identify the voice channel or channels which are not beingused for voice service, and, in response thereto, said datacommunication subsystem provides data service on one or more of saididentified voice channels.
 10. The cellular telephone system inaccordance with claim 8, wherein each said basestation monitoring meansmonitors use of said voice channels by said voice basestation toidentify the voice channel or channels on which voice service has begun,and, in response thereto, each said data basestation terminates dataservice on said identified channels.
 11. The cellular telephone systemin accordance with claim 8, wherein each said basestation monitoringmeans includes means for detecting at least one of a plurality of signalcharacteristics of an output signal of the associated voice basestationon each of a plurality of voice channels, and, in response thereto, eachsaid data basestation broadcasts an available-channel signature toindicate voice channels available for data transmission.
 12. Thecellular telephone system in accordance with claim 11, wherein saidsignal characteristics comprise the power of said output signals. 13.The cellular telephone system in accordance with claim 11, wherein saidremote unit includes remote unit monitoring means for monitoring each ofa plurality of selected voice channels for an available-channelsignature; a controller coupled with and responsive to said remote unitmonitoring means for selecting one of the voice channels identified bythe signature thereon as being available for data communication, and aremote unit transceiver coupled with said controller for transmittingdigital information over that channel.
 14. The cellular telephone systemin accordance with claim 13, wherein said remote unit further includesremote unit memory means for storing data identifying a plurality ofselected data-allocated channels corresponding to said control channels,and said controller is coupled with said remote unit memory means fordetermining which of said control channels carries the strongest signal,and using said stored data to identify at least one of the voicechannels corresponding to said control channel that is to be monitoredfor an available-channel signature.
 15. The cellular telephone system inaccordance with claim 14, wherein said controller is responsive to saidmonitored available-channel signature for up-dating said data stored insaid remote unit memory means with information obtained from saidavailable-channel signature.
 16. The cellular telephone system inaccordance with claim 13, wherein said remote unit monitoring meansmonitors said selected voice channel for an available-channel signatureprior to each of a plurality of data transmissions from said remote unittransceiver.
 17. The cellular telephone system in accordance with claim16, wherein said signal characteristics comprise radio-frequency powerlevels of the signals on each of said channels.
 18. The cellulartelephone system in accordance with claim 13, wherein said remote unitmonitoring means monitors a plurality of said selected voice channelsfor available-channel signatures thereon, and said remote unittransceiver transmits data over one of said selected voice channels onlyupon said remote unit monitoring means receiving one of saidavailable-channel signatures over said one selected voice channels. 19.The cellular telephone system in accordance with claim 8, wherein eachsaid data basestation monitoring means monitors at least one of aplurality of signal characteristics on one or more of a plurality ofvoice channels over which said associated voice basestation transmits,to determine which of said voice channels is not then in use by saidvoice communication subsystem, and each said data basestation furtherincludes a transceiver, which, in response to said monitoring meansbroadcasts an available-channel signature over one or more of said voicechannels to indicate that one or more of said voice channels areavailable.
 20. A cellular telephone system comprising:A) a voicecommunication subsystem including at least one voice basestation forproviding voice service over a plurality of voice channels; and B) adata communication subsystem including at least one data basestation forproviding data service over one or more of said voice channels; C)wherein said voice basestation includes a voice basestation transceiverfor transmitting a control signal over at least one control channel anda plurality of voice signals over said voice channels; said voicecommunication subsystem providing said voice service independently ofthe data service of said data communication subsystem; D) wherein saiddata communication subsystem includes basestation monitoring means formonitoring the voice service of said voice communication subsystem, adata basestation transceiver for transmitting data signals over saidvoice channels, and processor means responsive to said basestationmonitoring means for controlling said data communication transceivermeans so as to avoid having said transmitted data signals interfere withsaid voice service; and E) wherein said data communication subsystemfurther includes a plurality of data remote units, and wherein each saiddata remote unit includes remote unit monitoring means for monitoringeach of a plurality of selected voice channels for an available-channelsignature, a controller coupled with said remote unit monitoring meansfor selecting one of the channels identified by the signature thereon asbeing available for data communication, and a transceiver fortransmitting and receiving digital information over that channel. 21.The cellular telephone system in accordance with claim 20, wherein saiddata remote units further include remote unit memory means for storingdata identifying at least one data-allocated channels.
 22. The cellulartelephone system in accordance with claim 21, wherein said controllerup-dates the data stored in said remote unit memory means in response toinformation received in said available-channel signature.
 23. A remoteunit for data communication with a data basestation over a selected oneof a plurality of data-allocated voice channels of a cellular telephonesystem comprising a plurality of voice basestations, each voicebasestation associated with one of said data basestations and a controlchannel, said remote unit comprising:transceiver means for transmittingand receiving digital signals; and means for scanning the data-allocatedvoice channels to detect an available-channel signal transmitted by saiddata basestation that indicates voice channels that are not being usedfor voice communication and are therefore available for datatransmission, and controller means coupled with said transceiver meansand said scanning means for identifying, in response to saidavailable-channel signal, said voice channel to be used for datacommunication, and causing said transceiver means to transmit saiddigital signals over said voice channel so identified for datacommunication; and memory means for storing data identifying selecteddata-allocated channels corresponding to said control channels, andwherein said controller means is coupled with said memory means andresponsive to said available-channel signal for determining which ofsaid control channels carries the strongest signal, and for using saidstored data to identify the voice channels corresponding to said controlchannel that are to be scanned for said available channel signal. 24.The remote unit in accordance with claim 23, wherein said controllermeans, up-dates said data stored in said memory means with informationobtained from said available-channel signal.
 25. A cellular telephonesystem including a plurality of voice remote units equipped for voicecommunication, a plurality of data remote units equipped for datacommunication, a plurality of voice basestations, and a plurality ofdata basestations, said cellular telephone system providing primaryvoice communication between said voice remote units and said voicebasestation, and data communication between said data remote units andsaid data basestations, each said voice and data remote unit and saidvoice and data basestation including at least one transceiver, andwhereinA) said system temporarily allocates a plurality of voicechannels for packet-switched digital data service; B) each said databasestation being associated with a corresponding one of said voicebasestations, and further includes means coupled with said basestationtransceiver for causing said basestation transceiver to transmit, fromtime to time, at least one available-channel signature identifyingthose, if any, of the data-allocated channels associated with the databasestation that are not being used by said corresponding voicebasestation for voice communication and are therefore available for datacommunication; and C) each said data remote unit further includes datatransmission control means coupled to said remote-unit transceiverincludingi) controller means coupled with said remote unit transceiverfor selecting one of said data basestations with which the data remoteunit is to establish data communication, said one data basestation beingthe selected data basestations; and ii) means coupled with saidcontroller means for monitoring the data-allocated channels associatedwith the selected data basestation for available-channel signatures;said controller means selecting one of the channels identified by theavailable-channel signature thereon as being available for use in datacommunication with said data basestation, said one channel being theselected channel; and causing said transceiver of said data remote unitto transmit data on said selected channel to said selected basestation.26. A cellular telephone system in accordance with claim 25, whereineach said data remote unit further comprises a memory coupled to saiddata transmission control means for storing a look-up table identifyingthe selected channel associated with said data basestations.
 27. Acellular telephone system in accordance with claim 26, wherein each saiddata basestation includes processor means for causing said basestationtransceiver to transmit to said data remote units information for use bysaid controller means in up-dating the contents of the look-up table.28. A cellular telephone system in accordance with claim 25, whereinsaid controller means causes said remote unit transceiver to terminatetransmission of data after a pre-selected period of time.
 29. A cellulartelephone system in accordance with claim 25, wherein each said voicebasestation is associated with a control channel over which said voicebasestation transmits control signals, and said data transmissioncontrol means of each data remote unit selects said data basestation byidentifying one of said control channels on the basis of signalstrength.
 30. A cellular telephone system in accordance with claim 29,wherein each said data remote unit further comprises memory meanscoupled to said data transmission control means for storing a look-uptable identifying the selected data-allocated channels corresponding toeach control channel, and thereby to each data basestation, and saiddata transmission control means monitors the data-allocated channelsassociated with the selected basestation for available-channelsignatures.
 31. A method for transmitting data over a cellular telephonesystem including a plurality of remote units equipped for voicecommunication, a plurality of remote units equipped for datacommunication, a plurality of voice basestations, and a plurality ofdata basestations, said cellular telephone system providing primaryvoice communication between said voice remote units and said voicebasestation, and for communication between said data remote units andsaid data basestations, each said voice and data remote unit and saidvoice and data basestation including at least one transceiver, each ofsaid voice basestations associated with a corresponding data basestationand a control channel over which said voice basestation transmitscontrol signals, said method comprising the steps ofA) temporarilyallocating a plurality of voice channels for packet-switched digitaldata service; B) transmitting, from time to time, from each said databasestation, an available-channel signature identifying those, if any,of the data-allocated channels associated with said voice basestationcorresponding to said data basestation which are then not being used forvoice communication and are thereby available for data communication; C)performing, on a data-call-by-data-call basis, the following sub-stepsin each said data remote uniti) selecting one of said data basestationswith which the data remote unit is to establish data communication; ii)monitoring the data-allocated channels associated with the selectedbasestation for available-channel signatures; iii) selecting one of thechannels identified by the signature thereon as being available for usein data communication with said data basestation; and iv) causing saidtransceiver of said data remote unit to transmit data on said selectedchannel to said selected basestation.
 32. The method for transmittingdata over a cellular telephone system in accordance with claim 31,further comprising the step of maintaining look-up tables in said dataremote units identifying selected data-allocated channels associatedwith each of said control channels, and wherein saidbasestation-selecting step includes the steps of obtaining the identityof the data-allocated channels from the look-up table corresponding tothe strongest of the control signals, and saiddata-allocated-channel-monitoring step includes the step of monitoringonly the identified, stored data allocated channels.
 33. The method fortransmitting data over a cellular telephone system in accordance withclaim 32, wherein the look-up-table-maintaining step includes the stepof up-dating the contents of the table with information provided fromtime to time by the data basestations.
 34. The method for transmittingdata over a cellular telephone system in accordance with claim 31,further including the step of terminating data transmission oncommencement of voice communication over the selected channel by thecorresponding voice basestation.
 35. A cellular telephone systemcomprising:A) a circuit-switched communication subsystem for providingcircuit-switched communication service over a plurality ofradio-frequency channels; and B) a data communication subsystem forproviding packet-switched data communication service over one or more ofsaid radio-frequency channels; C) said circuit-switched communicationsubsystemi) using at least one control channel in association with saidcircuit-switched communication service; and ii) providing saidcircuit-switched communication service independently of thepacket-switched data communication data service of said datacommunication subsystem; and D) said data communication subsystemincluding means for monitoring the circuit-switched communicationservice of said circuit-switched communication subsystem, and processormeans for, in response thereto, controlling said packet-switchedcommunication data service to avoid interfering with saidcircuit-switched communication service; and E) wherein said means formonitoring the circuit-switched communication service of saidcircuit-switched communication subsystem monitors use of saidradio-frequency channels by said circuit-switched communicationsubsystem to identify the radio-frequency channel or channels on whichcircuit-switched service has begun, and, in response thereto, terminatesdata service on said identified channels.
 36. The cellular telephonesystem in accordance with claim 35, wherein said data communicationsubsystem identifies radio-frequency channels on which circuit-switchedservice has begun by monitoring power levels of signals produced by saidcircuit-switched communication subsystem.